[WiP] Validate password does not match any username

[SouradeepSaha]

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Description

Validate password does not match any username

Currenly, simple_password_check plugin only checks that a password does not match the username who's password is being set. However, the password can be set to another user in the table. Thus, implement logic to prevent password from matching any username in the table.

All new code of the whole pull request, including one or several files that are either new files or modified ones, are contributed under the BSD-new license. I am contributing on behalf of my employer Amazon Web Services, Inc.

Release Notes

A system variable, simple_password_check_usernames would be added with the allowed values ON/OFF, which would allow customers to turn this feature on/off. The following page would need to be updated: https://mariadb.com/kb/en/simple-password-check-plugin/

How can this PR be tested?

MTR tests will be added to verify the functionality of the new changes.

Basing the PR against the correct MariaDB version

• [x] This is a new feature and the PR is based against the latest MariaDB development branch.

PR quality check

• [x] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.